1. Field of the Invention
The invention relates to a balanced voltage-current converter, comprising a first and a second transistor whose collectors constitute a first and a second collector terminal, respectively, for a load circuit connected to a common first power supply terminal and whose emitters are each connected to a common second power supply terminal by means of a current supply element, said emitters constituting inputs of the voltage-current converter.
The invention also relates to a double-balanced mixer circuit comprising such a voltage-current converter.
2. Description of the Related Art
A voltage-current converter of this type having a load circuit in the form of a mixer circuit connected thereto is known from European patent application EP-Al-0131337 and is used, inter alia, in radio and television receivers and in receivers for mobile communication. The radio frequency (RF) signal from, for example, an antenna is applied in phase opposition to the input, terminals of the first and second transistors of the voltage-current converter and converted thereby into a proportional current variation which is superimposed via the first and second collector terminals, respectively on the emitter currents of the transistors of two difference pairs connected thereto. A mixer signal applied to these difference pairs switches their transistors so that signals having the sum of and the difference between the frequencies of the mixer signal and the radio-frequency signal are present on the collectors of the difference pairs. The signal having the difference between the frequencies, which is an intermediate frequency (IF) signal, can be taken off by means of a bandpass or lowpass filter for further processing.
In such a mixer circuit non-linearities produce intermodulation products and noise which directly influence the dynamic range of the receiver. The dynamic range which is free from intermodulation is defined as the difference, expressed in dB, between the minimum signal power required for noticeable reception and the maximum signal power for which reception, which is not interfered with by intermodulation products, is possible. It has been found that the quality of the voltage-current converter mainly determines the quality of the mixer circuit, that is to say, the dynamic range.
In the mixer circuit known from the above-mentioned European patent application EP-Al-0131337, the first and second transistors of the voltage-current converter are arranged in a common base configuration. The noise and distortion produced by the mixer circuit are largely determined by these transistors and the impedance of the signal source arranged between the input terminals of the voltage-current converter.
The dynamic range of the mixer circuit can be increased by increasing the bias current of these transistors so that the ratio between the impedance of the RF signal source and the impedance of these transistors, viewed at the input terminals of the voltage-current converter, is increased so that these transistors are driven to a relatively small extent. However, this solution has the drawback that the impedance of these transistors is not constant, which also causes distortion. Moreover, the bias current in integrated circuits cannot be increased without problems because it is usually limited to a predetermined value in connection with dissipation requirements.